1. Filed of the Invention
The present invention relates to a ridge waveguide semiconductor laser and, more particularly, to improvements in the optical output characteristic and high-frequency characteristic of a ridge waveguide semiconductor laser based on AlGaInAs/InP semiconductor material.
2. Description of the Related Art
The AlGaInAs/InP laser which has good temperature characteristic, is lately viewed as a promising element as the semiconductor laser used in communications based on optical fibers. FIG. 18 is a sectional view showing an example of the AlGaInAs/InP laser of the prior art. It is difficult to make the AlGaInAs/InP laser in an embedded structure since Al is included in the active layer as shown in FIG. 18. Therefore, a ridge waveguide configuration is generally employed. In FIG. 18, numeral 1 denotes an n-type InP substrate, 2 denotes an n-type InP cladding layer, 3 denotes an n-type AlInAs cladding layer, 4 denotes an n-type AlGaInAs light confinement layer, 5 denotes an AlGaInAs quantum well layer, 6 denotes a p-type AlGaInAs light confinement layer, 7 denotes a p-type AlInAs cladding layer, 10b denotes a p-type InP residue layer, 10a denotes a ridge portion comprising the p-type InP layer, 11 denotes a p-type InGaAs contact layer, 12 denotes an SiO2 insulation layer, 13 denotes a p-type electrode (Au) of the laser, and 14 denotes an n-type electrode (Au/Ge/Ni/Au).
The ridge waveguide semiconductor laser shown in FIG. 18 has been produced in the process shown in FIGS. 19A through 19F in the prior art. First, for example, the MOCVD method is employed to form the n-type InP cladding layer 2, the n-type AlInAs cladding layer 3, the n-type AlGaInAs light confinement layer 4, the AlGaInAs quantum well layer 5, the p-type AlGaInAs light confinement layer 6, the p-type AlInAs cladding layer 7, and the p-type InP layer 10 and the p-type InGaAs contact layer 11, laminated successively on the n-type InP substrate 1 as shown in FIG. 19A.
Then, as shown in FIG. 19B, a SiO2 insulating layer 20 is formed that is etched away in a photolithography process while leaving a portion where a ridge is to be formed. The SiO2, insulating layer 20 is used as a mask in dry etching or wet etching of the p-type InGaAs contact layer 11, and the p-type InP layer 10 is partially etched away leaving a part of the thickness, thereby forming the ridge portion 10a and the residue layer 10b of the InP layer 10 as shown in FIG. 19C.
Then as shown in FIG. 19D, the SiO2 insulating layer 20 is removed by etching followed by the formation of the SiO2 insulating layer 12, and only the portion on top of the ridge 10a is etched away in the photolithography process shown in FIG. 19E. Then the p-type electrode 13 and the n-type electrode 14 of the laser are formed as shown in FIG. 19F
In the production process described above, since the depth of etching of the p-type InP layer 10 is governed by time, depth of etching is likely to vary among lots, among wafers and among positions in the wafer surfaces. Consequently, there have been significant variations in the semiconductor laser characteristics among lots, among wafers and among positions in the wafer surface.
Variations in the semiconductor laser characteristics caused by the variations in the depth of etching are as follows. In case the p-type InP layer 10 is etched to a small depth, current 22 flowing beside the ridge increases, resulting in an increase in the threshold current, as shown in FIG. 20A. Also because a light emitting region 23 expands, the angle of spread of light in the horizontal direction decreases. In case the p-type InP layer 10 is etched to a greater depth, on the other hand, the current 22 flowing beside the ridge decreases, resulting in a decrease in the threshold current. Also because the light emitting region 23 becomes narrower, the angle of spread of light in the horizontal direction increases.
To restrict variations in the depth of etching of the p-type InP layer 10, Japanese Laid-Open Patent Publication No. 11-54837 discloses a method wherein a p-type GaInAsP etching, stopping layer 9 is provided below the InP layer 10 as shown in FIGS. 21A and 21B. The p-type GaInAsP etching stopping layer 9 is not etched by common etchants that are used in wet etching of the InP layer 10. Therefore, depth of etching of the InP layer can be kept constant by stopping the etching of the InP layer 10 at the GaInAsP etching stopping layer when the GaInAsP etching stopping layer is provided below the InP layer 10.
However, the AlGaInAs/InP laser of the prior art has the following problems.
First, as described in the Japanese Laid-Open Patent Publication No. 11-54837, in case the p-type GaInAsP etching stopper layer is provided on the p-type AlInAs cladding layer, the energy band at the junction of the p-type AlInAs layer and the p-type InGaAsP etching stopping layer has a discontinuous structure, so serial resistance becomes higher and the threshold current of the laser increases. As described in the Japanese Laid-Open Patent Publication No. 11-54837, the serial resistance can be decreased to some extent by optimizing the composition of the InGaAsP etching stopper layer, although it is not possible to completely suppress the increase in the serial resistance due to the discontinuous band structure even if this method is employed.
Light emitted from the AlGaInAs active layer 5 of the AlGaInAs/InP laser is spread around the light emitting region 23. In the conventional laser structure, the spread of light reaches the p-type electrode 13 as shown in FIG. 4A. The metal of the p-type electrode has a very high absorption coefficient for light of wavelengths in a range from 1.3 to 1.55 xcexcm that are the wavelengths of light emitted by the AlGaInAs/InP semiconductor laser. As a result, there occurs an absorption loss due to the p-type electrode which, in turn, causes an increase in the threshold current and a decrease in the light emission efficiency of the laser, thus deteriorating the laser characteristics.
Moreover, since a PN junction region (a region where the p-type layer and the n-type layer adjoin each other: the AlGaInAs quantum well layer 5 in the case of this example) expands laterally to portions where current does not flow, there has been a poor high-frequency characteristic of the semiconductor laser. This problem will be described below with reference to FIG. 22. FIG. 22 is a schematic diagram showing an equivalent circuit of the ridge waveguide semiconductor laser of the prior art. Current supplied through the p-type electrode 13 flows through a resistor 25 consisting of the p-type cladding layer 10, a diode 26 consisting of the quantum well layer 5 and a resistor 27 consisting of the n-type cladding layer 2, and enters the n-type electrode 14. A parasitic capacitance is parallel to the main current path, and causes deterioration in the high-frequency characteristic of the laser. The parasitic capacitance consists mainly of a capacitance 28 formed by the SiO2 insulating layer 12 in a pad portion and a capacitance 29 formed by the PN junction of the AlGaInAs quantum well layer 5 in a region where current does not flow. The capacitance 29 formed by the PN junction is far larger than the capacitance 28 of the pad. Since these components of capacitance are connected in series, total amount of the parasitic capacitance is determined by the capacitance 28 of the pad that has the smaller value. Consequently, roughly speaking, the capacitance 29 formed by the PN junction has a relatively small effect on the high-frequency characteristic of the laser. However, since the capacitance 29 formed by the PN junction is connected to the main current path via the p-type AlInAs cladding layer 7 (which constitutes a resistor component), the smaller the resistance of the p-type AlInAs cladding layer 7, the greater the effect of the capacitance 29 formed by the PN junction, and the lower the high-frequency characteristic.
The present invention has been made to solve the problems described above, and an object thereof is to provide a ridge waveguide semiconductor laser that is excellent in the optical output characteristic and high-frequency characteristic by reducing the threshold current of the laser and suppressing the influence of the parasitic capacitance caused by the PN junction region.
According to the first aspect of the present invention, a ridge waveguide semiconductor laser comprises:
a first conductivity type InP substrate;
a first conductivity type cladding layer of AlxGayIn1xe2x88x92xxe2x88x92yAs (0 less than x, 0xe2x89xa6y, x+y less than 1) formed above said InP substrate;
an active layer of Alx1Gay1In1xe2x88x92x1xe2x88x92y1As (0 less than x1, 0xe2x89xa6y1, x1+y1 less than 1) formed above said first conductivity type cladding layer,
a second conductivity type cladding layer of Alx2Gay2In1xe2x88x92x2xe2x88x92y2As (0 less than x2, 0xe2x89xa6y2, x2+y2 less than 1) formed above said active layer; and
a first InP cladding layer of the second conductivity type formed above said second conductivity type cladding layer;
an etching stopper layer formed above said first InP cladding layer; and
a second InP cladding layer of the second conductivity typte formed above said etching stopper layer, said second InP cladding layer having a ridge shape.
Since the InP cladding layer of the second conductivity type is formed via the etching stopper layer, there are less variations in the depth of etching when forming the ridge of the InP cladding layer of the second conductivity type. Moreover, since the etching stopper layer is formed over the Alx2Gay2In1xe2x88x92x2xe2x88x92y2As cladding layer via the InP layer, there occurs no problem of series resistance due to the discontinuous band structure between the etching stopper layer and the Alx2Gay2In1xe2x88x92x2xe2x88x92y2As cladding layer. As a result, variations in the threshold current and in the angle of horizontal spread of light among lots, among wafers and among positions in the wafer surface are restricted and it is made possible to provide a ridge waveguide semiconductor laser having reduced threshold current and excellent optical output characteristic.
The etching stopper layer of the second conductivity type is preferably formed from InaGa1xe2x88x92aAsbP1xe2x88x92b (0 less than a less than 1, 0 less than b less than 1). The etching stopper layer made of InaGa1xe2x88x92aAsbP1xe2x88x92b has better crystallinity, when formed on the InP layer, than when formed directly on the Alx2Gay2In1xe2x88x92x2xe2x88x92y2As cladding layer. The better the crystallinity is, the better a function to stop the etching step becomes.
A method of producing the ridge waveguide semiconductor laser according to the first aspect of the present invention comprises the steps of:
(a) forming said first InP cladding layer on said second conductivity type cladding layer;
(b) forming said etching stopper layer on said first InP cladding layer;
(c) forming said second InP cladding layer on said etching stopper layer;
(d) etching said second InP cladding layer by dry-etching vertically downward midway in the thickness thereof, while leaving a region where the ridge is to be formed; and
(e) etching said second InP cladding layer further vertically down to said etching stopper layer using an acid mixture including hydrochloric acid and phosphoric acid, thereby forming the ridge shape.
Use of the acid mixture including hydrochloric acid and phosphoric acid in the etching of the InP layer of the second conductivity type makes it possible to stop the etching step at the etching stopper layer, because of great difference in the selective etching rate between the InP layer and the etching stopper layer such as Alx2Gay2In1xe2x88x92x2xe2x88x92y2As (0 less than x2, 0xe2x89xa6y2, x2+y2 less than 1) (the InP layer is etched at a high rate while the Alx2Gay2In1xe2x88x92x2xe2x88x92y2As layer is etched at a very low rate). Therefore, variations in the depth of etching the InP cladding layer among lots, among wafers and among positions in the wafer surface can be eliminated by setting the duration of etching longer than the average time required to completely etch away the p-type InP layer. Also because etching with the acid mixture of hydrochloric acid and phosphoric acid proceeds only downward through the substrate in the last stage without hardly proceeding in the lateral direction, the InP layer can be etched vertically downward thereby to form a vertical side faces of the ridge.
According to a second aspect of the present invention, a ridge waveguide semiconductor laser comprises:
a first conductivity type InP substrate;
a first conductivity type cladding layer of AlxGayIn1xe2x88x92xxe2x88x92yAs (0 less than x, 0xe2x89xa6y, x+y less than 1) formed above said InP substrate;
an active layer of Alx1Gay1In1xe2x88x92x1xe2x88x92y1As (0 less than x1, 0xe2x89xa6y1, x1+y1 less than 1) formed above said first conductivity type cladding layer,
a second conductivity type cladding layer of Alx2Gay2In1xe2x88x92x2xe2x88x92y2As (0 less than x2, 0xe2x89xa6y2, x2+y2 less than 1) formed above said active layer;
an InP cladding layer of the second conductivity type formed above said second conductivity type cladding layer, wherein said InP cladding layer has a ridge shape splayed like a skirt near the base; and
a metal electrode formed above said InP cladding layer, said metal electrode formed along the ridge of said InP cladding layer.
Forming the InP cladding layer in the ridge shape having the portion near the base splayed like a skirt makes it possible to keep the metal electrode from the light emitting region and restrict the absorption of light by the metal electrode. As a result, the ridge waveguide semiconductor laser having a high external quantum efficiency can be provided. The reason for splaying only the portion near the base of the ridge for keeping the metal electrode is that splaying the entire side face of the ridge into a simple trapezoidal shape results in smaller area on top of the ridge that would increase the contact resistance between the metal electrode and the cladding layer (or the contact layer).
A method of producing the ridge waveguide semiconductor laser according to the second aspect of the present invention comprises the steps of:
(a) forming said InP cladding layer on said second conductivity type cladding layer of Alx2Gay2In1xe2x88x92x2xe2x88x92y2As;
(b) etching said InP cladding layer by dry etching vertically downward midway in the thickness thereof, leaving a region where the ridge is to be formed; and
(c) etching said InP cladding layer obliquely toward said second conductivity type cladding layer of Alx2Gay2In1xe2x88x92x2xe2x88x92y2As using an acid mixture including hydrochloric acid and phosphoric acid.
The InP layer can be formed into a skirt shape splayed downward in the portion near the base by applying dry etching midway through the InP cladding layer and etching the remainder further with the mixture of hydrochloric acid and phosphoric acid. While a vertical side face is finally formed when the InP is etched with the mixture of hydrochloric acid and phosphoric acid, increasing the proportion of hydrochloric acid in the mixture (for example, 1:1 for hydrochloric acid and phosphoric acid) causes, first, a sloped side face to be etched and then a vertical side face to be formed. Consequently, sloping etched side face can be formed to make the p-type InP layer of a splayed configuration, by increasing the proportion of hydrochloric acid and properly controlling the etching time.
According to the third aspect of the present invention, a ridge waveguide semiconductor laser comprises:
a first conductivity type InP substrate;
a first conductivity type cladding layer of AlxGayIn1xe2x88x92xxe2x88x92yAs (0 less than x, 0xe2x89xa6y, x+y less than 1) formed above said InP substrate;
an active layer of Alx1Gay1In1xe2x88x92x1xe2x88x92y1As (0 less than x1, 0xe2x89xa6y1, x1+y1 less than 1) formed above said first conductivity type cladding layer, wherein said active layer has a high resistance region interposing a main current path in said active layer;
a second conductivity type cladding layer of Alx2Gay2In1xe2x88x92x2xe2x88x92y2As (0 less than x2, 0xe2x89xa6y2, x2+y2 less than 1) formed above said active layer;
an InP cladding layer of the second conductivity type formed above said second conductivity type cladding layer, said InP cladding layer having a ridge shape; and
a metal electrode formed above said InP cladding layer.
By increasing the resistance of the active layer in a region interposing the main current path, it is made possible to decrease the parasitic capacitance formed by the PN junction of the active layer where current does not flow. Thus the ridge waveguide semiconductor laser of excellent high-frequency characteristic can be provided.
Further in the ridge waveguide semiconductor laser of the third invention, increasing the resistance of the Alx2Gay2In1xe2x88x92x2xe2x88x92y2As cladding layer of the second conductivity type in region interposing the main current path in the Alx2Gay2In1xe2x88x92x2xe2x88x92y2As cladding layer of the second conductivity type makes it possible to increase the junction resistance between the parasitic capacitance formed by the PN junction and the current path, thereby further improving the high-frequency characteristic of the ridge waveguide semiconductor laser.
The region of the Alx1Gay1In1xe2x88x92x1xe2x88x92y1As active layer of which resistance is increased is preferably the region that interpose the portion of the InP cladding layer of the second conductivity type right below the ridge. This configuration makes it possible to increase the resistance of the Alx1Gay1In1xe2x88x92x1xe2x88x92y1As active layer by using the ridge portion of the InP cladding layer as a mask.
However, since the high resistance region of the Alx1Gay1In1xe2x88x92x1xe2x88x92y1As active layer has a high absorption coefficient of light, the high resistance region may decrease the effective optical output when located near the light emitting region. In order to avoid this problem, when the resistance of the Alx1Gay1In1xe2x88x92x1xe2x88x92y1As active layer is increased in the region that interpose the portion of the InP cladding layer of the second conductivity type right below the ridge, it is preferable to form the InP cladding layer of the second conductivity type in a ridge shape with the portion near the base splayed like a skirt. This configuration makes it possible to decrease the overlapping area between the light emitting region and the high resistance region of the active layer, thereby decreasing the absorption loss of light. Also when the metal electrode is formed along the ridge of the InP cladding layer of the second conductivity type, absorption loss of light due to the metal electrode can also be decreased, thereby improving the external quantum efficiency of the laser.
Also in order to further restrict the absorption of effective light in the high resistance region, the high resistance region of the Alx1Gay1In1xe2x88x92x1xe2x88x92y1As active layer may be located apart from the area right below the ridge. This constitution has such an advantage as the high resistance region can be kept away from the light emitting region thereby eliminating the absorption loss of light, thereby improving the external quantum efficiency of the laser, although this configuration requires a somewhat complicated process to increase the resistance.
However, keeping the high resistance region of the active region away from the area right below the ridge, it is likely for the threshold current to increase due to the current flowing on both sides of the ridge. In order to suppress the threshold current increment, it is further preferable to increase the resistance of the Alx2Gay2In1xe2x88x92x2xe2x88x92y2As cladding layer of the second conductivity type in the region that interposes the portion of the InP cladding layer of the second conductivity type right below the ridge. By increasing the resistance of the Alx2Gay2In1xe2x88x92x2xe2x88x92y2As cladding layer of the second conductivity type beside the ridge, the spread of current flowing below the ridge is restricted, thereby decreasing the threshold current of the laser.
A method of producing the ridge waveguide semiconductor laser according to the third aspect of the present invention comprises the steps of:
(a) forming said second conductivity type cladding layer of Alx2Gay2In1xe2x88x92x2xe2x88x92y2As on said active layer;
(b) forming said InP cladding layer on said second conductivity type cladding layer;
(c) forming said InP cladding layer in a ridge shape having the portion near the base thereof splayed like a skirt; and
(d) increasing the resistance of said active layer by using said InP cladding layer as a mask.
By carrying out such processes as ion injection into the Alx1Gay1In1xe2x88x92x1xe2x88x92y1As active layer by using the InP layer of ridge shape as the mask, it is made possible to increase the resistance of the Alx1Gay1In1xe2x88x92x1xe2x88x92y1As active layer in a simple process. Also because the InP layer used as the mask has the ridge shape splayed downward, it is made possible to decrease the overlapping area between the light emitting region and the high resistance region, thereby decreasing the absorption loss of light due to the high resistance region.
In this patent specification, xe2x80x9cfirst conductivity typexe2x80x9d and xe2x80x9csecond conductivity typexe2x80x9d refer to different types of conductivity, p type or n type. When the first conductivity type is n type, the second conductivity type is p type and, when the first conductivity type is p type, the second conductivity type is n type.